Team 228 has two scales for weighing robots/parts, a 150-lb scale and a 400-lb scale. Here's a link to the 400-lb scale (which is the one we primarily use) that we have, which Staples sells for $150.
http://www.staples.com/webapp/wcs/st...plesZipCodeAdd
Like the previous posters, we just add on two cross members of aluminum and re-zero the scale, then weigh the robot.